The manufacturing of shaped bodies from fine-particle powdered materials is known, wherein these are mixed with plastifying and bonding agents and are shaped into "blanks," from which the plastifying and bonding agents are removed through heating before the powdered particles are sintered together by heating them to sintering temperatures, resulting in the desired formed body. Ferrite permanent magnets are also manufactured with sintering methods. Based on the publication of G. Heimke and J. D. Nye, Powder Metallurgy International, Vol. 5, No. 1, 1973, pages 28 ff., the powders are pressed while dry or wet (meaning in watery suspension), wherein the best magnetic qualities are achieved through wet pressing in the magnetic field. Furthermore, according to the aforementioned publication, the ferrite particles can also be incorporated into plastic materials. The magnets made from these plastic-bonded ferrite particles are normally produced through extrusion or injection-molding. Nothing is said about the type of plastic materials. This type of magnet is not sintered, but only cured.
In the DE-A1-36 26 360, an injection-molding process for manufacturing anisotropic permanent magnets is described. In that case, a "permanent-magnetic powder" is mixed with plastic materials and the mixture is granulated. The granulate is injected into the desired mold in the magnetic field, the shaped bodies are sintered and, if necessary, reworked. Finally, the sintered shaped bodies are magnetized. Mentioned as plastic materials obviously functioning as plastifying and bonding agents are "polyamides, polyurethanes, polypropylene, polyethylene, polystryrene, etc," without providing reproducible data of the characteristic values that can be achieved with this.
In the DE-C1-40 33 952, a binary binder system along the lines of the solid polymer solutions is proposed for processing metal powder through injection-molding. The production of magnets based on this method is not mentioned. The binary binder system consists of polyethylene and/or polypropylene as polymeric binder component, as well as cyclododecane, cyclododecanon, cyclododecanol and/or stearyl alcohol as low-molecular binder component.
In EP-A-0 115 104, a method for processing sintered, anorganic shaped bodies is described, which is also not focused on the production of magnets. Carbides, nitrides, borides and sulfides are named as base materials in addition to metallic and oxidic materials. The formed bodies are produced through injection-molding or extruding. One essential characteristic of the process is the use of a mixture, consisting of an oxidized wax and stearic acid as plastifying and bonding agent.
When producing permanent magnets according to the sintering method, it is desirable in the interest of good mechanical and magnetic qualities to produce shaped bodies with high packing density, uniform distribution and useful orientation of the powder particles. For one of the known methods, the wet pressing, the magnetizable particles that are as a rule added as a watery dispersion are pressed into a blank in a magnetic field. The particles then have an anisotropic orientation, which is advantageous for the later magnetizing of the sintered shaped bodies. Following pressing, the particles are initially demagnetized again, because magnetized blanks without plastifying and bonding agents are mechanically very sensitive. As a result of this sensitivity, the blanks must be demagnetized while still in the mold. However, the blanks remain relatively sensitive even after demagnetization, which complicates their handling in particular for the production on an industrial scale. On the other hand, wet-pressed blanks without plastifying and bonding agents have a comparably low tendency to form cracks during the sintering.
In contrast, blanks that are produced with another known method, that is by adding organic plastifying and bonding agents through deformation under magnetic orientation, have sufficient mechanical stability for the further processing and are therefore easy to handle even before the sintering. Like the wet-pressed blanks, they are as a rule demagnetized prior to the sintering. However, in contrast to the wet-pressing, this can take place outside of the mold because of their higher mechanical stability, which permits a more speedy operating sequence. On the other hand, the organic plastifying and bonding agents must be removed from the blank prior to the sintering. If this is not done with sufficient care, the blank disintegrates due to developing gas and the particles will not sinter later on. The "removal of the bonding agents" is a critical production step, which should be kept as short as possible for economic reasons. Finally, the blanks produced by using organic plastifying and bonding agents have a higher tendency to form cracks during the sintering than the wet-pressed ones.